1. Field of the Invention
The invention relates to compositions comprising mixtures of polysiloxane-polyoxyalkylene copolymers containing modified polyoxyalkylene building blocks and to the use of these compositions, as stabilizers, in the production of polyurethane foams.
Polysiloxane-polyoxyalkylene block copolymers are used in the production of polyurethane foams. They make possible the formation of a uniform pore structure and stabilize the foam during the production process.
Not all polysiloxane-polyoxyalkylene block copolymers can be used for this application. Their ability to be used is influenced by the equilibrium between the polysiloxane blocks and the polyoxyalkylene blocks in the copolymer, which has to be set with precision. The structure of the two blocks is therefore of great importance. The structures of the blocks can be varied using a large number of parameters.
The composition of the polyoxyalkylene block can be altered in terms of the oxyalkylene units present; particular examples are oxyethylene, oxypropylene and oxybutylene. By composition in this context is meant not only the relative number but also the arrangement of the oxyalkylene units. Furthermore, the molecular weight and the end group of the polyoxyalkylene block can be varied. Linkage between the polyoxyalkylene block and the polysiloxane can be by way of a hydrolysis-stable Si--C bond or a less hydrolysis-stable Si--O--C bond.
The polysiloxane can be altered in terms of the number of siloxane units, the branching, and the number of linkage possibilities to the polyoxyalklene.
The effectiveness of a polysiloxane-polyoxyalkylene copolymer in foam stabilization can be predicted only to a very limited extent. The expertise in this field therefore lies in an empirical study of the effects of the possible structural alterations.
2. Description of the Related Art
Polysiloxane-polyoxyalkylene block copolymers are described in numerous publications. The following citations therefore represent only a selection.
DE-A-15 70 647 is concerned with the Si--O--C linkage of chloropolysiloxanes to polyoxyalkylene units of which from 40 to 70% have molecular weights of from 1000 to 3000, preferably having secondary OH groups, and up to 50 OH equivalent percent have a molecular weight of from 130 to 3500, and whose structure consists of ethylene oxide and/or propylene oxide. Preference is given to a molecular weight of up to 1750.
DE-A-16 94 366 relates to the use of polysiloxane-polyoxyalkylene copolymers having the following structure as foam stabilizers:
from 25 to 70% of the weight consist of a polyoxyalkylene having an average molecular weight of from 1600 to 4000, with an ethylene oxide content of from 20 to 100% by weight, the other monomer, if present, being propylene oxide or, optionally, higher alkylene oxides, and from 30 to 75% by weight of a polyoxyalkylene having an average molecular weight of from 400 to 1200, with an ethylene oxide content of from 65 to 100% by weight, the other monomer, if present, being propylene oxide or, optionally, higher alkylene oxides. PA1 R.sup.2 are R.sup.1 and/or R.sup.3 and/or R.sup.4 wherein at least one R.sup.2 is not R.sup.1, PA1 a is a number from 1 to 100 if b is a number from 6 to 8, PA1 a is a number from 1 to 200 if b is a number from 3 to 6, PA1 a is a number from 1 to 300 if b is a number from 0 to 3, PA1 b=0 to 8, PA1 subject to the condition that in at least one of the silicone copolymer components of the mixture both e and f are 1 and in at least one of the silicone components of the mixture either e or f is 0.
EP-A-0 275 563 describes a block copolymer which is built up of three different polyoxyalkylene units. One block contains from 20 to 60% by weight of oxyethylene units and has a molecular weight of from 3000 to 5500; a further block contains from 20 to 60% by weight of oxyethylene units, with a molecular weight of from 800 to 2900; and a third block consists only of polyoxypropylene and has a molecular weight of from 130 to 1200.
Despite the large number of siloxane-polyether block copolymers which are thus obtainable, the large number of fields of use for these active substances--for example, as additives for both solventborne and low-solvent coating materials, inks and pastes, for coating textiles and paper (tissues), as a starting material for crosslinking reactions, as antifoams, as emulsifiers, as additives to cosmetic preparations, as stabilizers for the wide field of PU foam production (firstly owing to the large number of possible type of polyurethane foam, such as flexible foam, rigid foam, ester foam, cold foam, packaging foam, flame-laminatable foam, molding foam, integral foam, etc., secondly owing to new techniques of foaming, such as variable pressure foaming, foaming with pressurized inert gases, or else forced cooling processes, e.g., Enviro-cure, Crain Industries)--makes it necessary to look for further-improved structures.
Apart from the large number of possible combinations of various siloxane chains with different degrees of modification and different sites of modification with polyether side chains of various molecular weight, EO-PO ratio, different end groups, possibly also in combination with other modifying groups on the siloxane, a fundamental difference between stabilizers used is the nature of the preparation of the corresponding polyether-modified siloxanes. Generally, two routes are described:
First, the synthesis of the hydrolysis-stable Si--C bond by means of a hydrosilylation reaction between a siloxane containing SiH groups and an olefinically substituted polyether, or the synthesis of a less hydrolysis-stable Si--O--C bond by means of the linking of chloro-substituted siloxanes to hydroxyl-functional polyethers. The attachment of substituents to a polysiloxane can be performed by two different kinds of chemical reaction. By hydrolysis of chlorosiloxanes with polyethers, with elimination of hydrogen chloride, an Si--O--C bond is formed. The preparation of these compounds can be found in German Patents 10 12 602, 10 40 251 and 11 20 147 and in U.S. Pat. No. 3,115,512.
The second possibility is the so-called hydrosilylation reaction in which the radicals R.sup.3 and R.sup.4 are introduced onto the molecule of the polysiloxane by an addition reaction onto SiH groups of the polysiloxane in the presence of a hydrosilylation catalyst, with the formation of an Si--C bond. According to the prior art, platinum catalysts, such as, for example, cis-dichlorodiammineplatinum(II) or hexa-chloroplatinic(IV) acid, are used for this second possibility, which is described in U.S. Pat. No. 2,846,458 and in the German Laid-open and Examined Documents 12 20 615 and 11 53 166.
In the preparation of polyurethanes, these substances are usually used as uniform substances or else as a mixture of silicone polyether copolymers of the same preparation type. In some cases (in ester foam, for example) the silicone-polyether copolymers used are also combined with other surface-active substances in order, for example, to improve the emulsion effect of the additives.
It has surprisingly now been found that the mixing of different silicone polyether copolymer types, i.e., the use of both Si--C- and Si--O--C-linked silicone polyether block copolymers in one mixture, permits particular combinations of properties. Particularly noteworthy in this context is the possible combination of activity and cell fineness.